In a wireless communication system environment there is a need for the establishment of secure communications that is resistant to natural interference, jamming, disturbances, interceptions, interferences, and detection.
One method for achieving secure communications within a wireless environment is the use of a modulation technique such as direct sequence spread spectrum (DSSS), where the transmitted signal takes up more bandwidth than the information signal that is being modulated. The carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. Direct Sequence Spread Spectrum (DSSS) employs a pseudorandom noise (PN) binary sequence (interchangeably referred to as “PN sequence” or “PN spreading sequence” herein) to spread-modulate payload data. For binary phase shift keying (BPSK) spread modulation, the PN sequence is a real number sequence, while for quadrature phase shift keying (QPSK) spread modulation, the PN sequence is a complex number sequence occupying both in-phase and quadrature channels. A PN sequence created using PN sequence generators determines statistical properties of a “noise-like” spreading pattern of the signal across the allotted spectrum. For this method to successfully transmit and receive radio signals, both transmitter and receiver must use an identical pseudorandom PN spreading sequence. This signal is transmitted on a bandwidth considerably larger than the frequency content of the original information.
A spread-spectrum transmission offers advantages over a fixed-frequency transmission, the transmissions are highly resistant to narrowband interference, are difficult to intercept, and can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications in the shared frequency band. As a result, bandwidth can be utilized more efficiently.
PN sequence generators are some of the most critical components in DSSS communications. They generate PN binary sequence with desirable statistical properties. A PN sequence generator traditionally generates a sequence of pseudorandom binary numbers using a linear-feedback shift register (LFSR) structure. In BPSK spread modulation, only one LFSR is needed to generate a real number sequence, while in QPSK spread modulation, two independent LFSRs are required to generate complex number sequences. The LFSR is implemented using several shift registers in tandem with feedbacks selected from various stages of register outputs. A PN sequence generated from a LFSR is solely determined by the LFSR feedback logic connections as represented by a polynomial (interchangeably referred to as LFSR feedback polynomial or PN sequence generator polynomial) and the initial register values in LFSR (interchangeably referred to as seed or initial seed). LFSR feedback polynomial and seed used by a transmitter must be identical to those used at a receiver in order to synchronize DSSS communication. For the purpose of information security, LFSR feedback polynomials and seeds used by a transmitter and receiver pair need to change over time to generate time-varying PN sequences, and for DSSS communication synchronization, requires that changes at the transmitter and receiver must be time-synchronized in lock step.
For security purposes, cryptographic algorithms running at each node of a pair of nodes generate identical non-linear, non-repeating spreading sequences. These cryptographic algorithms require seeds that are known at each node and are identical between any communication pair of nodes. Cryptographic algorithms prove to be very complex, costly and require a large form factor in order to provide the requisite security.
In addition to the PN sequence used for spread modulation of payload data, a preamble sequence is needed at the start of each packet in packet-based DSSS communications. A preamble sequence possesses certain statistical properties to facilitate preamble detection and packet synchronization at a receiver node. For secured DSSS communications, preamble sequences must change from packet to packet, i.e., time-varying. Moreover, the change of preamble sequences must be synchronized in lock-step between a pair of communicating nodes.
In DSSS communications, concatenated sequences have been commonly used as preamble sequences. A concatenated sequence is constructed from a real-numbered outer code sequence and a real- or complex-numbered inner code sequence. For example, For BPSK spread modulation, preamble sequences are constructed from real-numbered inner code sequences; while for QPSK spread modulation, preamble sequences are constructed from complex-numbered inner code sequences, corresponding to in-phase and quadrature channels. Typically, the outer code sequence used to construct preamble sequence is selected a priori from sequences with small side lobes in a-periodic autocorrelation function. Small side lobes help to reduce the probability of false synchronization.
The preamble sequence generation and the PN sequence generation are generally treated separately. For commercial DSSS communication systems, fixed preamble sequences and non-time-varying PN sequences are commonly used. These sequences lack security consideration because of their non-time-varying nature. For military DSSS communication systems, complicated cryptographic algorithms are used to generate non-linear non-repeating sequences for payload data modulation. Although the non-linear non-repeating sequence is highly secure, the communication system is complex with an extra device that hosts cryptographic algorithm.